Aerosol dispensers yield optimum performance when used within preferred operating temperature ranges. Performance problems associated with lower-than-preferred operating temperatures include sputtering, decreased uniformity and/or range of spray, shorter open times, and degraded operational properties of aerosolized materials such as adhesives. On the other hand, storage, transportation and/or use of aerosol dispensers at higher-than-preferred temperatures may result in excessive internal pressure leading to rupture of the dispenser, as well as causing excessive overspray and/or chemical degradation of the material to be aerosolized. Preferred temperature ranges thus depend in part on characteristics of the material to be aerosolized and any propellant present to pressurize the liquid in an aerosol dispenser. Such characteristics include viscosity, chemical stability, vapor pressure, specific heat and surface tension.
Notwithstanding the sensitivity of aerosol dispensers"" performance and safety to operating temperature, many such devices are used with little or no reference to such temperature. For example, the operating temperature of an aerosol dispenser may or may not be closely related to ambient temperature. Temperatures achieved after equilibration of an aerosol dispenser for significant periods in relatively hot or cold locations may not be changed quickly on movement to an area of different ambient temperature. Further, in a dispenser that is partially-filled with liquid, the space above the liquid will usually equilibrate more rapidly than the liquid itself. Heat from the hand of a user may raise the temperature of the upper portion of a dispenser significantly while the temperature of liquid in the lower portion of the dispenser may have changed very little. Users who rely on the xe2x80x9cfeelxe2x80x9d of an aerosol dispenser may thus be deceived as to whether the dispenser (particularly the liquid to be aerosolized) has reached an acceptable operating temperature.
Other temperature-related problems with aerosol dispensers can occur when the dispenser is unwittingly placed on a heat sink (e.g., a concrete floor or a large piece of metal) which may result in relatively rapid changes in the temperature of liquid in the dispenser. And still other problems stem from the fact that expansion of the propellant gas within the dispenser during use is a cooling process. While cooling may occur relatively quickly in the gaseous space above the pressurized liquid during use, the liquid itself will usually cool more slowly. So again, users who rely on the xe2x80x9cfeelxe2x80x9d of an aerosol dispenser may believe that the liquid to be aerosolized has reached an unacceptably low operating temperature substantially before it actually has. Conversely, an aerosol dispenser in heavy use may be cooled sufficiently by gas expansion to require repeated adjustments of measured temperature by adding heat to the dispenser periodically during such use. The possibility of user miscues in these situations could be reduced or eliminated if reliable and timely estimates of the liquid temperature in an aerosol dispenser could be inexpensively made. But conventional bulb thermometers are relatively fragile and not readily adaptable for use with aerosol dispensers. And infrared thermometers, in addition to being fragile, are too expensive for routine use with the wide variety of aerosol dispensers in common use.
The present invention relates to methods and apparatus for making better aerosols by effectively addressing the temperature-related problems noted above. One begins with a container having inner and outer surfaces, adding liquid to the container and pressurizing the liquid. Using the present invention, one estimates the liquid temperature by measuring the container outer surface temperature in an area directly or nearly directly opposite a portion of the container inner surface that contacts the pressurized liquid. By comparing the measured surface temperature with a predetermined (preferred) temperature range and then controlling heat flow to the container to adjust the measured temperature to within the predetermined range, the pressurized liquid is prepared for release from the container through a nozzle as an aerosol. Note that the controlled heat flow may be positive (i.e., heat transfer into the container and liquid from a heat source such as hot water for raising their temperatures), negative (i.e., heat transfer out of the container and liquid to a heat sink such as cold water for lowering their temperatures), or zero (i.e., when the container and liquid temperatures are within the predetermined range and no heat transfer is required).
In preferred embodiments of the present invention, a liquid crystal temperature indicator is either permanently or reversibly adhered to the outer surface of an aerosol dispenser in a location that will allow estimation of the temperature of the liquid inside the dispenser. Liquid crystals are composed of elongated organic molecules that can exhibit different physical properties (e.g., optical and electrical properties) at different temperatures. Using, for example, changes in the color of a plurality of liquid crystals at different temperatures arranged in numerical (i.e., ascending or descending) order, temperature indicators of the present invention can be coupled to aerosol dispensers to indicate desired temperature adjustments to a dispenser within a range of temperatures. The temperature indicators thus act as guides for the use of appropriate heat flow control methods for achieving preferred temperature conditions for making and using aerosol. United States Patents related to temperature measurement using liquid crystals include Nos.: U.S. Pat. No. 4,064,872 (Capon), issued Dec. 27, 1977; U.S. Pat. No. 6,257,759 (Winston, et al), issued Jul. 10, 2001; U.S. Pat. No. 6,294,109 (Raton, et al); and U.S. Pat. No. 6,284,078 (Witonsky, et al), issued Sep. 4, 2001, each said patent incorporated herein by reference.
Because liquid tends to collect in the lower portion of a dispenser intended to be stored and/or operated vertically, a temperature indicator will preferably be adhered to the external surface of the dispenser bottom, or to an adjacent portion of the external surface of the dispenser wall lying between and spacing apart the top and bottom of the dispenser. Since hand-held dispensers of this general design are often for single or short-term use in light duty applications, preferred embodiments of the temperature indicators for such dispensers may incorporate a non-permanent self-adhesive backing allowing the indicator to be peeled off one dispenser and applied to another.
In commonly-available hand-held aerosol dispensers (e.g., those used for small quantities of spray paint or insect repellant), the dispenser is usually operated and stored in a vertical position, so that any liquid present in the dispenser collects in the bottom portion. A finger-operated valve actuator and nozzle assembly is sealed in the top of the dispenser. The valve actuator and nozzle assembly, in turn, is sealingly coupled to an internal valve that controls flow to the nozzle from a liquid feed tube extending from the dispenser top into the liquid to be aerosolized. To allow use of virtually all of the liquid in a dispenser, the liquid feed tube usually extends to or nearly to the bottom of the dispenser, with one or more ports for liquid entry to the feed tube, all such ports remaining under the liquid surface during all operational conditions when the dispenser is held substantially vertically.
On the other hand, special-purpose aerosol dispensers may have sizes, shapes and service lifetimes very different from the hand-held dispensers described above. For example, certain commercial models may weigh up to 18 kilograms. And some aerosol dispensers may comprise a canister adapted for repeated refilling with liquid to be aerosolized. Where repeated and/or extended use of a dispenser is anticipated, more robust methods of adhering the temperature indicators of the present invention to the dispenser surface are preferred. These methods may include, but are not limited to, permanent adhesives, mechanical clamps, and elastic bands. Further, transparent protective coatings and/or films may be applied over temperature indicators to reduce damage due to rough handling while still allowing visual access to the indicator.
Regardless of where and how they are placed on an aerosol dispenser, temperature indicators of the present invention are adhered to the dispenser surface on which they are placed so as to reliably reflect the temperature of that surface accurately, preferably within about 1.11 degrees Celsius (C.). In certain applications where, for example, a rough or uneven dispenser surface might impede accurate surface temperature measurement by a liquid crystal temperature indicator of the present invention, a heat-conducting medium such as a gel may be applied between the indicator and the surface to improve adhesion between surface and indicator.
Additionally, certain aerosol dispensers may have shapes that allow or even mandate placement of a plurality of liquid crystal temperature indicator(s) at various locations on the dispenser outer surface instead of the placement on or adjacent to the bottom as described above. For example, where convection currents in the liquid to be aerosolized are impeded by dispenser geometry and/or properties of the liquid (e.g., high viscosity), a plurality of temperature indicators may be needed to indicate significant temperature gradients in the liquid. If present, such temperature gradients may indicate the need for additional thermal equilibration time and/or control of heat flow to the dispenser and liquid. In each such instance, temperature indicators will preferably be placed to indicate surface temperature of an exterior portion of the aerosol dispenser that is directly or nearly directly opposite a portion of the dispenser interior wall contacting the liquid to be aerosolized.